Magnetic resonance imaging (MRI) using nuclear magnetic resonance (NMR) are known. Under NMR and MRI, a magnetic field is applied to a sample (e.g., tissue). In response, the magnetic moments of the nuclei of certain molecular components of the sample (e.g., a proton of a water molecule) may align themselves with the magnetic field. Protons of water molecules will be used as an example in the following, since this is the signal used for conventional clinical magnetic resonance imaging.
Once aligned with the magnetic field, a radio frequency may be applied to the proton magnetic moment which perturbs the moment away from its equilibrium state. The magnetic moment then begins to induce a current in a detector coil tuned to the resonance frequency. This process is referred to as free induction decay (FID).
The frequency at which a given magnetic moment will resonate is a function (among other things) of the type of molecular component involved, its chemical bonds, the molecular environment and the magnetic field applied to the component. The resonance frequency is typically referred to as the Larmor frequency.
Where a magnetic field varies across a sample, a proton at one location has a different Larmor frequency than at another location. In knowing a magnetic field gradient across a sample, a Larmor frequency can be calculated at each position. In knowing the Larmor frequency at each position, an excitation frequency may be selected which will excite protons and produce signal at only one specific location. Using a variety of frequencies corresponding to different protons across a magnetic field, a magnetic resonance signal may be detected from protons in each one of a number of slices. Additional gradients are applied to obtain two dimensional images of each slice, based on the amount of signal detected at each resonance frequency within the excited slice.
One difficulty producing images based on applied magnetic field gradients is that the applied gradients are not the only fields which control the Larmor frequency. For example, the frequency of the proton FID may vary due to a number of factors (e.g., chemical environment, interactions with surrounding protons and other magnetic moments, and local magnetic susceptibility gradients, etc.). These effects, which are typically referred to as resonance offset effects cause distortions in magnetic resonance images, which can only be corrected if the resonance offset effects are directly measured.
Conventional magnetic resonance imaging procedures typically do not detect the entire proton spectrum of the FID in each three-dimensional space (voxel) and as a result, the resonance offset effects referred to above are not measured. This lack of spectral information has several drawbacks:
First, the incomplete spectral information results in a loss of anatomical information due to artifacts related to resonance offset effects.
Second, due to the incomplete capture of spectral information, the information contained in the line shape and resonance frequency of fat and water is lost. If this information could be captured, it could be used to improve image contrast, and might be clinically useful.
Third, conventional MR imaging relies primarily on T.sub.1 and T.sub.2 relaxation to provide image contrast. While, T.sub.2 * contrast might have great clinic utility, it is not practical to obtain strongly T.sub.2 * weighted contrast (i.e., using gradient echo images). For example because of the lack of spectral information, T.sub.2 * contrast is typically accompanied by severe artifacts due to resonance offset effects, even when spatial resolution is very high.
Previous efforts to improve the performance of MRI have incorporated low resolution spectroscopic information into MR imaging solutions, using methods suggested by W. T. Dixon, and their variants. These approaches have met with some success in improving image quality. These previous efforts rely on incomplete spectra of water and fat, however, and therefore do to take full advantage of FID spectral information. Accordingly, a need exists for an MRI system that takes full advantage of spectral information that may be acquired from fat and water molecules. This can be done using spectroscopic imaging methods based on those proposed by Maudsley et al. and Brown et al. However, in practice, image acquisition with these methods is too slow for most clinical examinations. Therefore, this invention discusses the use of fast spectroscopic imaging methods similar to those proposed by Mansfield et al. To obtain high resolution spectral and spatial information. FSI methods make it practical to acquire spectroscopic image data in a short amount of time, i.e., within the time allotted for a normal clinical exam.